Abstract: This is the fifth chapter of the book Corporations in 100 Pages (2020), authored by Holger Spamann, Scott Hirst, and Gabriel Rauterberg. The book is an introduction to corporate law for students and anyone else interested in the foundations of corporate law. The book provides an accessible, self-contained presentation of the field’s essentials: what corporations are, how they are governed, their interactions with their investors, and other stakeholders, major transactions (M&A), and parallels with other legal entities, including partnerships. Optional background chapters cover the investor ecosystem, contemporary corporate governance, and corporate finance. The book’s exposition of doctrine and policy is nuanced and sophisticated, yet short and simple enough for a quick read. Chapter 5 explains the law governing “Fiduciary Duties,” which are legal duties imposed on specific individuals (“fiduciaries”) who exercise power on behalf of others. The chapter first provides an overview of corporate fiduciary duties: who owes what to whom, and introduces the principal fiduciary duties of care and of loyalty. The chapter then distinguishes standards of conduct and standards of review, and explains the two main standards of review that apply to fiduciary duties in corporate law, the “business judgment rule” and “entire fairness.” The chapter then explains how these standards of review apply to the paradigm cases of self-dealing and mere carelessness, as well as to cases involving corporate opportunities, bad faith, knowing violations of law, and (lack of) candor or oversight.